The present invention relates to a polymethacrylic acid ester (hereinafter referred to as "methacrylate") resin composition which is excellent in processibility and is capable of providing a film having excellent weatherability, transparency, flexibility and resistance to whitening upon contact with hot water, and a process for preparation thereof. More particularly, the present invention relates to a cross-linked polyacrylic acid ester (hereinafter referred to as "acrylate") elastomer-containing polymethacrylate resin composition comprising a grafted elastomer having a graft polymer and a free polymer (in the instant specification, the part which is grafted on the elastomer (A) is defined as a "graft polymer" and a copolymer wherein the graft polymer has been grafted on the elastomer (A) is defined as a "grafted elastomer"), produced by graft-polymerizing a monomer mixture comprising mainly a methacrylate in the presence of a cross-linked acrylate elastomer (A) mainly composed of an acrylate, in which the component ratio of the graft polymer of the grafted elastomer is substantially the same as that of the free polymer [hereinafter, the components used for preparing the graft part of the grafted elastomer and the free polymer, that is, the mixture to be added to the elastomer (A) for preparing the graft polymer and free polymer (resin component) are referred to as a "monomer component (B)"], and a process for preparing the composition.
Methacrylate resins are remarkably excellent in weatherability and transparency compared with other plastic resins. So, they are widely used in various fields such as the industrial field. For example, methacrylate resins are formed into sheets, films, and the like and they are laminated on various materials, such as plastics, woods and metals, to prevent deterioration of the materials, to keep their good appearance, and the like.
However, methacrylate resins are rigid and brittle. Therefore, various means have been proposed for improving these deficiencies.
In order to improve film-moldability and impact resistance of the films prepared from the methacrylate resins, there have been proposed various methods, e.g. a method in which a rubber component is dispersed in the methacrylate resin; a method in which in the graft-polymerization of a mixture of acrylate and methacrylate onto a rubber component, the percentage of the acrylate monomer in the monomer mixture is increased; a method in which components used for forming a graft polymer of the grafted elastomer and a free polymer is graft-polymerized in a multistage polymerization which improves the strength of the film (Japanese Unexamined Patent Publication No. 13371/1972, No. 9022/1975, and the like), and the like.
When the rubber component is mixed with, or dispersed in, the methacrylate resin for improving the above-mentioned deficiencies, generally, it is common to use a method in which monomers used for obtaining the methacrylate resin are graft-polymerized in the presence of the rubber for obtaining good compatibility of the methacrylate resin with the rubber.
As rubbers, there are preferably used an unsaturated rubber such as a butadiene rubber from the viewpoint of the improvement of the impact resistance. However, a saturated rubber, such as an acrylic rubber, is usually used as the rubber from the viewpoint of maintaining excellent weatherability, which is a feature of the methacrylate resin. The smaller the particle size of the rubber, the better the transparency but this lowers the impact resistance. Accordingly, the particle size of the rubber is suitably determined according to the use to which the product will be put.
In the case of the graft-polymerization of monomers used for obtaining methacrylate resins in the presence of the rubber, in order to improve entanglement adhesion between the rubber part and the graft polymer of the grafted elastomer and to change physical properties, such as refractive index, more continuously there have been proposed multistage polymerization methods such as a four-stage polymerization (Japanese Examined Patent Publication No. 13371/1972) and a three-stage polymerization (Japanese Examined Patent Publication No. 9022/1975), and the like. However, the multistage polymerization methods have dificiencies in that the manufacturing steps become complicated with an increase of the number of the steps. Also, when introducing a soft intermediate component between the rubber component and the graft polymer, it becomes difficult to obtain films having excellent whitening resistance to hot water.
Also, since the methacrylate resins have relatively high in hygroscopicity, the resins are easily whitened due to absorption of moisture when the films made from the resins are dipped in warm water or when the outdoor temperature rises after raining in case of using the films outdoors. Particularly, where the films are transparent, the transparency, which is the feature of the methacrylate resin, of the film is lowered, which remarkably impairs the value of the films in commerce.
For solving the above-mentioned defects, there has hitherto been studied, for instance, a method in which a hydrophobic monomer such as cyclohexyl methacrylate is used, a method, in which the graft ratio is increased for improving the adhesion between the rubber component and the graft polymer, a method in which a substance having water repellency or hydrophobic property, such as a silicon compound or a plasticizer, is kneaded with the methacrylate resin, thereby preventing the whitening of the film caused by absorbing warm water, or decreasing absorption of warm water. However, sufficient results have not yet been obtained.
On the other hand, there has been described, in Japanese Examined Patent Publication Nos. 14267/1977, 33277/1979, and the like, a method in which methacrylate monomers are graft-polymerized in the presence of a rubber in one stage without using a chain transfer agent. In these methods, although in some cases that a film having excellent whitening resistance to hot water is made, the resin is not suitable for extrusion, at a high draw ratio, to form a film. Therefore, the processing methods are limited.
An object of the present invention is to provide a cross-linked acrylate elastomer-containing methacrylate resin composition which can be converted into a film having excellent weatherability, transparency, flexibility and resistance to whitening when immersed in hot water, and which can be stably formed by extrusion, at a high draw ratio, into thin films having the above-mentioned excellent properties.
An another object of the present invention is to provide a process for efficiently preparing a methacrylate resin composition having excellent processibility and capable of providing a film having excellent properties such as weatherability, transparency, flexibility and resistance to whitening upon contact with hot water.
These and other objects of the present invention will become apparent from the following description.